1. Field of the Invention
The present invention relates to a semiconductor device that includes a sensor formed by use of an MEMS (Micro-Electro-Mechanical System) technique, and more particularly relates to a technique for enhancing the reliability of sensors.
2. Description of the Related Art
One conventional method for enhancing the reliability of a sensor formed by an MEMS technique is, for example, to passivate its top surface by forming a silicon nitride film, as disclosed in Japanese Patent Laid-Open No. 2006-156182. Specifically, according to the technique disclosed in this publication, a capacitive MEMS pressure sensor having a moving electrode (or a diaphragm electrode) and an opposing fixed electrode is formed on the top wiring layer of a semiconductor circuit device, and then the top layer including the MEMS sensor is passivated by forming a silicon nitride film thereon in order to enhance the environmental resistance of the device.
However, such a conventional technique is disadvantageous in that the sensor may exhibit drift in characteristics. One reason for this is that pinholes occur in the passivation film on the elevated step portions of the MEMS portion and, as a result, moisture penetrates into the MEMS sensor through these pinholes. Another reason is degradation of the quality of the films making up the MEMS sensor. In order to avoid these problems, a passivation film having a thickness of approximately one micron (such as used in conventional semiconductor integrated circuit devices) may be formed instead of the above passivation film (i.e., silicon nitride film). However, such an arrangement is difficult to apply to MEMS sensors, since it will greatly affect their operating characteristics. Further, in the case of pressure sensors, it is a challenge to maintain their reliability, since the pressure-sensitive portion formed on the semiconductor substrate must be directly exposed to the atmosphere.